Fuel cells are rapidly becoming a significant source of power in our society, and their use in a variety of applications is inevitable. One such application is the use of a fuel cell as a power source in an uninterruptible power supply (UPS) for use with an electronic device or digital equipment, such as a personal computer (PC).
One advantage of using a fuel cell instead of a battery as the power source in a UPS is the fuel cell's high energy density, and therefore, the ability to operate a system for very long periods of time while off the utility grid. However, a fuel cell based UPS is not without limitations. Although it can operate for very long periods of time, it is still limited by the amount of fuel (usually hydrogen) available.
Fuel cell based UPSs that have emerged typically operate in a parallel backup configuration to the utility grid and rely on status monitoring and control in combination with a transfer switch or the like to operate when the line power is interrupted. An exemplary prior art fuel cell based UPS is disclosed in U.S. Pat. No. 6,602,627 to Liu et al., the contents of which are expressly incorporated herein by referenced. Typical prior art fuel cell based UPSs generally require appropriate consideration for the hold time of the equipment to be powered and the transfer time of the UPS in switching over to fuel cell operation.
In a few fuel cell based UPSs that has emerged, a DC-AC inverter is typically incorporated for converting the fuel cell's DC power to AC power for use by the equipment to be powered. When use as power backup for a PC, for example, there may be a minimum of two power conversions that take place between the fuel cell and the computer's actual components. They include a DC-AC conversion in the UPS and an AC-DC conversion in the computer's power supply. These conversions waste a considerable amount of hydrogen in feeding the fuel cell and allowing the power produced by the fuel cell to dissipate as heat. Additionally, if the inverter in the power supply is incapable of accepting a wide input voltage range that a typical fuel cell provides, an additional DC-DC converter must be used to bring the input voltage to within the inverter's tolerance, which results in three inefficiencies.
Accordingly, there is a need for a fuel cell based true online UPS that has improved operating efficiency.